Al-Qaeda-linked groups that are enticing dozens of Australians to join the bloody conflict in Syria have begun setting up camps to train foreign fighters to launch terrorist attacks when they return home.

The alarming change in tactics, revealed by US intelligence chief James Clapper, comes as Australia's counter-terrorism ambassador Bill Fisher told Fairfax Media the threat of an attack on a bar, mall or another ''soft'' target frequented by Westerners had ''worsened'' in recent times.

Intelligence and law enforcement agencies have been grappling with a surprising resurgence of al-Qaeda over the past year, as its affiliates capture major cities in western Iraq and assume control of the military resistance in Syria.

As many as 200 Australians are believed to have travelled to Syria to help rebels trying to topple dictator Bashar al-Assad. Of those, a ''few dozen'' are actively engaged in combat, although authorities don't have a precise handle on the numbers.

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According to Mr Clapper, US intelligence has evidence of ''training complexes'' within Syria ''to train people to go back to their countries and conduct terrorist acts''.

Mr Clapper, who is US director of national intelligence, named Jabhat al-Nusra as one group with ''aspirations for attacks on the homeland''. He strongly suggested it wasn't the only one, but declined to provide any more detail on the camps.

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About half of the Australians fighting in Syria are believed to be members of Jabhat al-Nusra. Others have joined the Islamic State of Iraq and the Levant (ISIL), an even more militant Islamist group with strong ties to al-Qaeda.

US and Australian intelligence agencies have long worried about the possibility of their citizens returning battle-hardened from Syria, with the combat skills and virulent anti-Western ideology that might enable them to launch a terrorist act.

Those worst fears now appear to be well founded.

''This is a huge concern,'' said Mr Clapper, adding that he did not believe the danger of a terrorist attack was ''any less'' than what it was a decade ago.

The assessment of the level of threat was backed by Mr Fisher, a veteran diplomat who became Australia's counter-terrorism ambassador last year.

''The likelihood of an attack like 9/11 in the West has lessened but the capability of al-Qaeda and its affiliates to undertake lots of smaller but nonetheless deadly attacks is very real - hitting bars where Westerners congregate overseas, and other soft targets. In this respect, the threat is worse,'' he said. ''Any places where there is very low domestic security control is an obvious target.''

The attacks in Bali in 2002 claimed 202 lives, including 88 Australians. While counter-terrorism officials have made hundreds of arrests in the region, concerns remain that Australians and other Westerners could be targeted abroad, including at popular tourist destinations in south-east Asia.

While established terrorist networks have been disrupted, Greg Barton from the Global Centre for Terrorism Research said smaller self-starting terrorist cells are emerging, including in Indonesia, and are difficult to detect.

Intelligence agencies in Australia, he said, have also struggled to get a grip on who has travelled to Syria and what they are doing over there.

''The numbers [of Australians going to Syria] have caught everybody by surprise,'' he said, adding that they far exceeded the exodus of Islamists to other conflicts, including the wars in Afghanistan and Iraq.

Mr Clapper said al-Qaeda's affiliates have developed counter-espionage techniques that have confounded Western agencies. ''They've gone to school [on] us, on how we try to track them,'' he said.

''The combination of … the geographic dispersal and the increasing challenges in collecting against them makes al-Qaeda, in all of its forms, a very formidable threat.''